Allow adding auxiliary variables like Ramsey multipliers to var_list_

The auxiliary variables are endogenous variables like every other variable. A call like ramsey_policy(instruments=(i),irf=13,planner_discount=betta,periods=200) x pi MULT_1; would be suficient to display IRFs for the multiplier 1. However, the preprocessor does not allow adding MULT_1 to the variable list, because: Unknown symbol: MULT_1 We should allow adding any variable present in M_.endo_names to the var_list_. @houtanb Could you do this, please? Related to http://www.dynare.org/phpBB3/viewtopic.php?f=1&t=12117